1. Field of the Invention
This invention relates generally to marine seismic surveys and more particularly to a method and apparatus for synthesizing and analyzing the output response of an air-gun array and for displaying information to a user for real-time quality control of a seismic survey operation.
2. Description of the Related Art
In marine seismic surveying, to obtain geophysical information relating to the substrata located below the sea bottom, seismic sources, generally acoustic transmitters, adapted to produce pressure pulses or shock waves under water, are towed beneath the water surface behind a marine vessel. The shock waves propagate into the substrata beneath the sea where they are reflected back to the sea. Sensors (usually hydrophones) are used to detect the returning shock waves and to output signals indicative of the detected wave. The signals are processed to generate useful data and to determine the geophysical structure of the substrata.
Air guns or gas guns are frequently used as acoustic transmitters. Usually, several air guns are placed in spaced relation to each other in an array. One or more air gun arrays are towed behind a marine vessel beneath the sea surface. During operation, all air guns in an array are activated simultaneously to produce a desired overall pressure pulse from that array. The pulse characteristics, such as the frequency, bubble ratio and amplitude, of the overall pressure pulse produced by an air gun array is a function of the characteristics of the pressure pulses produced by the individual air guns and the physical arrangement of the air guns in that air gun array and of each gun in that array.
Usually, a shipboard central controller controls the array, and the controller is coupled to the array by an umbilical leading out to the array. Shipboard controllers have been improved over the years to help ensure simultaneous activation (or firing) of the air guns. One such system is described in U.S. Pat. No. 4,757,482 to Fisk and having the title “Modular Airgun Array Method, Apparatus and System”, the '482 patent. That patent describes an air gun control system having a central controller on the ship with a data bus leading to several sources aligned in an array and towed behind the ship. The controller of the '482 patent provides some in-water control features by the use of a plurality of local control modules that perform power conversion and are individually addressable by the shipboard central controller.
Marine seismic surveyors have several goals for managing energy source output. One goal is to maximize the energy output of the seismic source array. Another goal is to maintain the array operational characteristics within a predetermined set of specifications or limit conditions. Energy produced by a source array is maximized by maintaining the proper timing of array elements and by monitoring individual elements for out-of-tolerance conditions. The term “array” refers to multiple air guns activated simultaneously. The term “element” refers to a single air gun. The term source or acoustic source as used herein generically refers to either a single air gun or to an array of air guns.
Timing is problematic with typical source systems that control timing from the acquisition vessel. A telemetry cable that extends from the vessel to the source element acts as a filter in the system and it limits the operator's ability to precisely control element timing. Source elements that are not precisely timed will produce energy that interferes and reduces the overall array output. Moreover, data signals returning from hydrophone acoustic sensors will also suffer from the same imprecision.
System operators normally use assumptions about a source array signature when processing seismic data signals to recover the true reflectivity of the subsurface by suppressing distortions. The usual processing methods use deconvolution techniques, which are adversely affected when initial assumptions are inaccurate. Therefore, as an array output degrades due to timing or element errors, the initial assumptions become less accurate and thus reduce the reliability of the processed data signals.
Another problem with the typical prior art system is that element failure often reduces operational effectiveness. A failed source in an array adversely affects initial assumptions by changing the array combined output pulse. If detected, the operator might continue operation with a small number of failures, but this reduces data quality. Also, the operator might install spare elements in the array to activate subsequent to the failure, but this adds cost to the survey operation. Ultimately, the operator might be forced stop production to retrieve and repair the source array, resulting in significant efficiency losses.
These and other problems with the typical seismic survey system create a need for an apparatus and method for determining real-time an array health status from which the operator can make an informed real-time decision for continuing a survey with a failed element. As used herein, the term real-time means any course of action or activity during a seismic survey.
The typical system also suffers from an inability to provide information useful in predicting system response given a potential failure. Therefore, the need exists for predictive array synthesis that takes element failure into account. Such array synthesis will allow an operator to predict array performance with one or more elements removed from the array and to determine if the array would remain within specifications given the removed elements.
Yet another problem associated with the typical system is that the operator needs an improved interface for effectively controlling the array in view of potential failures. Current seismic survey systems do not provide a graphical user interface having real-time status reporting, quality control reporting, or troubleshooting tips for use during the survey.